This invention relates to web-advancing apparatus.
In particular, the invention relates to apparatus for advancing strip-shaped photographic material, for example photographic paper supplied from a roll.
It is known to supply photographic paper in strips of e.g. 500 m or 600 m length which are wound on a supply roll. This paper is withdrawn from the supply roll by a pull-off device--e.g. a pair of nip rollers--and fed into a storage device from where it is advanced toward a processing station in stepwise manner. In general the withdrawal of the paper from the supply roll, and its forwarding to the storage device, is carried out intermittently rather than continuously. For this purpose the drive for the nip rollers is frequently energized and deenergized which each time results in a sudden tug at the supply roll. Since the inertia of the very heavy roll resists the tug initially, the paper is briefly subjected to high tension and may even tear; neither possibility is desirable, for evident reasons.
To counteract this problem it is known to interpose a tilt arm between the supply roll and the nip rollers; this arm has a guide roller about which the paper web is made to travel. A spring acts upon the arm, allowing it to yield to tensional stresses by tilting in the direction from which the tension is applied. This is intended to counteract sudden tensional stresses and to allow the heavy supply roll to start turning gradually in response to pull exerted by the nip rollers.
It has been found, however, that this prior-art equipment does not fulfill the expectations set by it. The reason is that the mass (and inertia) of a full (fresh) supply roll and an almost empty one, differ greatly from one another. As a consequence the shock damping effect of the tilt arm is ineffective during withdrawal of the final third of the paper length from the roll, if the spring force acting upon the tilt arm is too great. Conversely, if the spring force is too small, then the tilt arm will not dampen any shock forces during the withdrawal of an initial length of paper from the roll.